Copper-niobium, copper-vanadium, or copper-chromium nanocomposites, and the use thereof in heat exchangers

ABSTRACT

We propose here a class of new materials for high heat-flux applications including high flux heat exchangers, rocket engines, jet engines, gas turbines, space-plane wings, and fusion reactors. The materials are nano-composites formed from copper and a refractory metal, especially niobium, vanadium, or chromium, but also potentially silver, iron, tantalum, tungsten, or molybdenum. The copper plus refractory mix is fast-melted, e.g. by arc melting, and then fast-cooled and worked. When cast the component metals separate into a fractile metal-metal composite that should have excellent heat-transfer qualities. Working the material makes it a lot stronger by extending the fractile structures into micron, and submicron (nano-scale) filaments and sheets of metal-metal composite. The resulting strong, high thermal-conductivity material should be excellent for demanding heat exchange applications, especially those where the heat flux is so high that ordinary materials of construction would suffer from thermal creep: that is from large forces generated internally by the differential expansion caused by the heat flux. Typical heat exchanger surfaces that might use this material might be tubes or indented flat plates.

BACKGROUND OF THE INVENTION

Of non-composite, ordinary materials, the ones that are most resistantto high thermal stress tend to be alloys of molybdenum and alloys ofcopper, see Table 1. These alloys excel in their thermal stressparameter, kσT/αE, combining high thermal conductivity, reasonably highstrength, low young's modulus, and low thermal expansion coefficients.Applications that need these properties include any highly compact, highvalue heat exchangers, as in rocket and jet engines, fusion reactorfirst walls, hypersonic plane wings, or any other high thermal stressand shock applications.

Some other high temperature alloys appear attractive, and can beexpected to exceed these two for very special applications. Based ontheir thermal stress parameters, other attractive materials shouldinclude Be, Mo—Re, Ta—W, and tungsten (W), but these materials sufferfrom being more expensive than Mo or Cu alloys, and/or suffer from beinghard to fabricate and join.

SUMMARY OF THE INVENTION

A heat exchanger is provided that is made of a copper-refractory metalcomposite that defines a structure of the heat exchanger. A process offorming a refractory nanofilament filled copper matrix is provided thatincludes fast casting a molten mixture of a refractory copper mixture ofmore than 10% refractory to form a casting. The casting is thenmechanical worked to form dendrites with mean diameters of less than 10μ(micron).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention involves the use of nano-composites of refractory metals(particularly Nb, V, and Cr) dispersed within a copper matrix. Inparticular, nanocomposites containing 10-60% refractory componentdispersed in the copper, and highly worked to form fractilenanofilaments and nano-thickness sheets dispersed in it. Wire of thismaterial was developed as an intermediate step on the process to makeniobium-tin superconducting wire, but a comparison of its predictedthermal shock parameter to those of Mo and copper alloys, Table 1,suggests that sheets and tubes of this nano-composite should be veryuseful for high thermal flux heat exchangers.

TABLE 1 Comparison of key parameters related to materials' thermalstress resistance Mo W V Nb Cu Cu—Cr Be Cu—40Nb Cu—40V structure bcc bccbcc bcc fcc fcc hcp nano-C nano-C density 10.28 19.25 6.11 8.57 8.92 8.81.85 8.78 8.36 MP ° C. 2620 3422 1915 2468 1085 1085 1287 1085 1085 k(W/m ° C.) 142 178 31 52.7 399 140 190 260.5 251.8 E (GPa) 320 407 128103 130 130 296 119 128 σT (MPa) 700 900 300 500 200 1000 300 1000 1000α (μ/m ° C.) 4.9 4.5 8.4 7.3 16.6 16.6 11.3 12.9 13.3 cost2004 $/kg 2016 45 60 2 10 800 50 40 kσT/αE 63.4 87.5 8.7 35.0 37.0 64.9 17.0 169.7147.7

As Table 1 shows, the thermal stress parameter of Cu-40% Nb is predictedto be more than double that of molybdenum or copper alloy due to ahigher critical strength, a lower Young's elastic modulus, and a higherthermal conductivity, predicted here based on a high electricalconductivity. Copper beryllium is a used in rocket-engine heatexchangers, a very high thermal stress application [2].

Although it is generally difficult to form nanometer-thickness wires andsheets in a metal matrix, the proposed nanocomposite essentiallyself-form when copper-niobium, copper-vanadium, or copper chrome is fastcast, e.g. following arc melting, and is then worked (deformed) to thinthe filaments and sheets. It appears that the best nano-composites formwith very pure, very ductile metals, e.g. pure niobium and copper athigh copper contents (over 82% Copper), but ductile nano-composites alsoform with copper and chromium, though chromium is not ordinarilyductile. We have also been able to make nano-composites with 40-50volume percent niobium or vanadium in copper, and that appears to be atechnical first.

One key requirement to make these nano-composites is that the twocomponents must be immiscible in each other in the solid phase; anotheris that the metals be very pure, and have a very low oxygen content ofless than 2 atomic % and preferably less than 0.5 atomic % and mostpreferably between 0.001 and 0.3 atomic %. Upon fast casting from themelt, these immiscible mixtures are observed to form a fractalinterconnecting networks of dendrites, 1μ to 100 nm in diameter. Inlow-oxide copper, these dendrites are very ductile. When the two-phasematerials is now drawn or rolled the copper and refractory dendritesthin out, reaching about 10-50 nm without needing an anneal step. Theresulting metal-metal composites can be quite strong [3,4] and should bequite thermally conductive. Because these composites self-form onprocessing, they are sometimes called “Deformation Processed Metal-MetalComposites” or DPMMCs as a shorthand [4].

The high thermal conductivity values in Table 1 are predicted by asimple rule of mixtures based on an assumption of interconnectingmatrixes, and the conductivity of the pure materials. Measurements ofthe electrical conductivity seem to confirm this simple rule—at leastfor lightly annealed composite along the direction of draw [4]. Based onthis, we can predict that the nano-composites will show the high thermalconductivity values in Table 1 when measured along the direction ofrolling, and not-much lower conductivity when measured across thedirection of rolling.

Table 1 also shows exceptional break-strengths for the Cu—Nb composite,and modest values of E, the Young's elastic modulus. These values goalong with the composite's very high strain to break, and very highstrain to initiate creep, about 3% for modestly worked material [3].These high strengths, and higher, up to 2200 MPa have been measured forhighly worked Cu-20% Nb, and for several other similar metal pairsincluding Cu-15% Nb, Cu-15% Ta, and Cu-15% Fe [4]. The values for Cu-40%Nb shown in Table 1 are projections based on an extrapolation betweenCu-20Nb, Cu-10% Nb, and Cu-15% Nb.

A possible explanation for the very high strength of the highly workedcomposite is that the niobium ends up as filaments that are only 10-25nm thick. This is the equivalent of 40-100 refractory atoms width.Similar strength has been seen in other metals when drawn down tofilament sizes this narrow. The copper also appears to be hardened inthe composite as the refractory prevents intercrystalline creep.

Cu-40% Nb, Cu-50% Nb, and Cu-45% V have been made for us by theMaterials Preparation Center of Ames Lab. All these compositions appearto be reasonably ductile; and the early experiments confirm that thehigher refractory compositions are a lot stronger than thelow-refractory ones. We expect this pattern to show particularly at hightemperatures, where there is the greatest need for new heat-exchangermaterials. Cu-60% Nb has been made as part of the invention, but it hasnot been tested yet, and may not be as useful for heat exchangerapplications as the thermal conductivity may suffer. It should benoticed that the measured Young's modulus for the modestly worked,Cu-20% Nb composite 128 MPa matches, reasonably closely to the value onewould expect from the values for copper and niobium, following a simplerule composition-weighted average.

Theory of Thermal Stress:

A material's resistance to mechanical force is shown below in terms ofthe force per unit area, σ, the material's Young's modulus, E, and thestrain ΔL/Lo:

F/A=σ=EΔL/Lo  1)

In SI units, s is measured in Pa, force in Newtons and area inmeters-square. In SI, E is likewise measured in Pa, or N/m2. For anunconstrained metal, the linear thermal expansion is:

ΔL=LoαΔT or ΔL/Lo=αΔT  2)

where α is the coefficient of thermal expansion, and ΔT is the change intemperature, ° C. When the surface of a material is heated across itsthickness, the hot surface is constrained to not expand more than thecenterline does. Canceling the required elastic strain, ΔL/Lo inEquation 1 against the thermal induced strain, ΔL/Lo from 2, we find:

σ=αEΔT  3)

the maximal amount of heat transfer, Q*, through a material is themaximal ΔT* to the centerline times k, the thermal conductivity, anddivided by the material thickness to the centerline, ∂.

Q*=ΔT*k/∂  4)

Combining Equations 3 and 4 we see that the maximal heat transfer of thematerial times this material thickness, ∂, is

∂Q*=σTk/αE  5)

where σT is the maximal strength of the material at the use temperature.

The key materials parameter in Equation 5 is kσT/αE. This is seen todetermine the robustness of a given thickness of material to thermalstress. For thin-walled tubes constrained at the ends, the thermalstress parameter can be shown to include the Poisson ratio:

M=2(1−ν)σTk/αE  6)

Where M is the thermal stress parameter and ν is the Poisson ratio.

Table 1 compares the parameter in Equation 5, kσT/αE, for a variety ofpossible first wall materials. It is seen that copper alloy, molybdenum,beryllium, and tungsten are the best of the simple materials inresistance to thermal stress. According to the predictions in Table 1,these composites should resist thermal far better than these, or anysimple alloy at a given wall thickness based on kσT/αE. These advantagesshould extend to fairly high temperatures, important e.g. for use in gasturbines and fusion reactor first walls.

The high strength of the these materials may allow for the use ofthinner walled heat-exchange surfaces than possible with lower-strengthmaterials. Thus, for example, allowing for the use of exceptionallysmall wall thickness tubes in heat exchangers. According to equation 6,this should reduce thermal stress further, and should allow formore-compact heat exchangers.

REFERENCES

-   1. B. Badger, R. W. Conn, etc., A High-performance Non-circular    Tokamak Power Reactor Design Study—UWMAK-III, UWFDM-150, University    of Wisconsin, 1976.-   2. A. M. Russell and K. L. Lee, Structure-Property Relations in    Nonferrous Metals, Wiley 2005.-   3. J. Bevk, J. P. Harbison, and J. L. Bell, “Anomalous Increase in    Strength of In Situ Formed Cu—Nb Multifilamentary Composites” J.    Applied Phys. 49 (1978) 6031-6038.-   4. A. M. Russell, L. S. Chumvley, and Y. Tian, “Deformation    Processed Metal-Metal Composites,” Advanced Engineering Materials,    2 (2000) p. 11-22.

1. A heat exchanger comprising a copper-refractory metal composite thatdefines a structure of the heat exchanger.
 2. The heat exchanger ofclaim 1, where the refractory metal is niobium
 3. The heat exchanger ofclaim 1, where the refractory metal is vanadium
 4. The heat exchanger ofclaim 1, where the refractory metal is chromium
 5. The heat exchanger ofclaim 1 where the composite is worked to a true-strain in excess of 2.6. The heat exchanger of claim 1, where the composite is worked to atrue strain in excess of
 5. 7. The heat exchanger of claim 1 where therefractory metal content is between 10% and 60%.
 8. The heat exchangerof claim 1, where the refractory content is between 20% and 50%
 9. Theheat exchanger of claim 1 as part of a rocket engine.
 10. The heatexchanger of claim 1, where the exchanger serves as a fusion reactorfirst wall.
 11. The heat exchanger of claim 1, where the exchanger ispart of a jet engine.
 12. The heat exchanger of claim 1 as part of a gasturbine system.
 13. The heat exchanger of claim 1 where the exchangesurface is in the form of a tube made of the composite.
 14. The heatexchanger of claim 1 where the exchange surface is in the form a flatsheet.
 15. Heat exchanger of claim 1, where the surface is coated with alayer of a metal of nickel, molybdenum, beryllium, or tungsten.
 16. Heatexchanger made of claim 1 where the refractory metal is tantalum,molybdenum, tungsten, iron, silver, or a combination thereof.
 17. Aprocess of forming a refractory nanofilament filled copper matrixcomprising: fast casting a molten mixture comprising a refractory coppermixture of more than 20% refractory to form a casting; then mechanicalworking said casting to form dendrites whose mean diameter is less than10μ.
 18. The process of claim 17 wherein said mechanical working isrolling or drawing.
 19. The process of claim 18 wherein said mechanicalworking is rolling or drawing and occurs without annealing.
 20. Theprocess of claim 17 wherein said dendrites have a mean diameter is lessthan 500 nanometers.
 21. The process of claim 17 wherein said dendriteshave a mean diameter of 40 to 400 atoms width and are formed of copper,refractory, or a combination thereof.
 22. The process of claim 17wherein said molten mixture consists essentially of copper with morethan 20% percent niobium, vanadium, or chromium or a combinationthereof.
 23. The process of claim 17 wherein said molten mixture has alow oxygen content.
 24. The process of claim 17 where said moltenmixture consists essentially of copper with more that 20% tantalum,molybdenum, tungsten, iron, or a combination thereof